1. Field of the Invention
The present invention relates to an electroluminescent element which emits light by applying a voltage to electrodes formed on upper and lower sides of a luminescent layer, and in particular relates to the structure of an electroluminescent element containing a switching element.
2. Discussion of the Related Art
An electroluminescent element has luminescent materials in a luminescent layer to which an electric field is applied which emits light by excitation when accelerated free electrons inside the luminescent layer collide with them.
FIG. 12 is a cross-sectional view of a conventional electroluminescent element wherein a lower electrode 102, a first dielectric layer 103, a luminescent layer 104, a second dielectric layer 106 and an upper electrode 107 are formed on an insulating substrate 101 in this order. The luminescent layer 104 contains luminescent materials in a matrix material and is entirely surrounded by the first dielectric layer and the second dielectric layer.
In the electroluminescent element which has a structure such as described above, rare earth metal fluorides are used as the luminescent materials. When a high electric field (for example, 2.0 MV/cm) is applied between the upper electrode 107 and the lower electrode 102, electrons jump out from the interface between the first dielectric layer 103 and the luminescent layer 104 or between the second dielectric layer 106 and the luminescent layer 104 into the luminescent layer 104, and are energized by being accelerated in the high electric field. These high-energy electrons collide with the luminescent materials contained in the luminescent layer 104 and excite the luminescent materials. The light emission arises when the excited luminescent materials return to their ground state.
Using a film forming method such as vapor deposition or sputtering, a large number of electroluminescent elements as described above can be formed on a large substrate to form a flat panel display.
An electroluminescent flat panel display has a plurality of electroluminescent elements arranged over the surface of a substrate. The lower and upper electrodes of these electroluminescent elements are linear and orthogonal, forming a matrix structure. The electroluminescent flat panel display also has a plurality of driver circuits for the electroluminescent elements. Supposing m represents the number of linear lower electrodes and n represents the number of linear upper electrodes, then the electroluminescent flat panel display requires (m+n) driver circuits in total.
When an AC voltage is selectively applied to the lower and upper electrodes by the driver circuits, the luminescent layer at the points of intersection of the electrode matrix to which the AC voltage is applied emits light, and accordingly, the required image can be displayed as a combination of the electroluminescent elements which emit light and those which do not.
However, to generate the high electric field capable to cause the light emission in the electroluminescent element mentioned above, a high voltage (for example, 200 V) must be applied to the upper and lower electrodes. Therefore, the driver circuits in the electroluminescent flat panel display have to be able to switch 200 V AC on and off, and the driver integrated circuits functioning as switching elements must be able to withstand this high voltage.
Driver integrated circuits able to withstand high voltages are expensive because of the particular process of manufacturing. In consequence, the problem occurs that the electroluminescent flat panel display is also expensive.
When electroluminescent elements arranged in a matrix are used for the display, light emission at each point occurs only once or twice during each frame in which the linear lower and upper electrodes are selected sequentially to scan all the picture elements. Therefore, the electroluminescent materials which emit red or blue light cannot be used as the emitting elements for a display because of their low intensity of light emission.